Synergy studies in combined injection of Lower Hybrid and Electron Cyclotron waves

The combined use of Lower Hybrid (LH) Waves and Electron Cyclotron Waves (ECW) has been theoretically proposed in order to improve the overall Current Drive (CD) efficiency of the two systems. The basic idea is that the ECW interact with the fast electrons produced by LH waves. This interaction can either increase the number of fast electron or increase the maximum energy of the tail. In both cases, the driven current associated with the fast electron tail is larger than the current separately obtained by employing the two systems.

Even though indications of this interaction have been previously observed in other tokamaks (as JFT-2M, TdeV, Versator II), clear macroscopic effects on the plasma state have never been reported due to a series of limitations (transient scenarios, low plasma density and therefore electron collision time of the fast electrons larger than the radial diffusion time, low ECW absorption). For the first time FTU has demonstrated that ECW power is efficiently absorbed by the fast electron tails generated by the LH waves (8 GHz). This occurs when no interaction between the thermal electrons and the ECW when the magnetic field (B= 7.2 T) is well above the resonant field with thermal electrons: with the FTU ECW system (140GHz), BRES=5T. Thus, at B=7.2T the wave can resonate only with supra-thermal electrons, such as those produced by LHCD. When both ECRH and LHCD are applied, there is a significant drop in the loop voltage that indicates an increase of the current drive efficiency as shown in Fig 1. Figure 2 shows the electron temperature radial profiles with LH alone (600 kW), with the first (350 kW) and second (700 kW) ECW power step. The increase occurs within r<10 cm (r/a=0.3) and is larger than 1 keV at the centre.

Fig. 2 - Thomson Te(r) profile at different times: Ohmic phase (red), LH (0.6 MW) only(violet), LH+ECRH (0.6+0.35 MW: green; 0.6+0.7 MW: cyan)

Main results:

- With LHCD alone very good current drive efficiency is achieved at high magnetic field (7.2T) and full current drive has been obtained with an LH power of about 1 MW, neo = 0.5 1020 m-3 and I=400kA. Central electron temperature reaches 4 to 6 keV.

- With ECRH only at B = 7.2 T, no effect on the plasma was observed.

- The main monitors for synergistic effects are the downshifted second harmonic radiation and the Fast Electron Bremmstrahlung (FEB) camera (in collaboration with CEA/Cadarache). The best synergy was observed when the fast electrons profile according to the FEB camera is peaked or flat. In this case, the downshifted ECE radiation temperature increases from 50 to 100keV during ECW injection.

- The importance of these results is in having demonstrated that the synergy between LH and ECH waves, both for heating and current drive, can be achieved at densities relevant for burning plasma. Moreover it extends the exploitation of the ECH system at much higher magnetic field values than the resonant ECRH magnetic field.

Experimental Reports